Variable damping-force damper and manufacturing method of the same

ABSTRACT

A variable damping-force damper includes a cylinder tube filled with magnetic particles (MRF), a piston that is slidably disposed within the cylinder tube, a piston rod connected with the piston and is disposed so as to protrude out of one end of the cylinder tube and a rod guide that closes one end of the cylinder tube and slidably supports the piston rod. The cylinder tube has a Ni plating film whose Vickers hardness is 800 VHN or more on its inner peripheral surface and the piston slides relative to the Ni plating film. The rod guide has a structure having a predetermined base material portion and a fluorine resin contained Ni plating film that is treated by heat and is provided on the surface of the base material portion. The piston rod slides relative to the fluorine resin contained Ni plating film.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. 119 ofJapanese Patent Application 2008-30763 filed on Feb. 12, 2008, JapanesePatent Application 2008-89065 filed on Mar. 31, 2008 and Japanese PatentApplication 2008-150528 filed on Jun. 9, 2008, the disclosures of whichare incorporated in this application by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable damping-force damper andmore specifically to a variable damping-force damper for use in dampingvibrations of a vehicle or the like for example as well as to amanufacturing method of the same.

2. Description of Related Art

There is known a variable damping-force damper using a MRF(Magneto-Rheological Fluid) as a working fluid in which sphericalparticles having an average size of around several μm and ferromagnetism(referred to as “magnetic particle” hereinafter) are dispersed as adispersed material in oil such as mineral oil, i.e., a dispersant medium(this variable damping-force damper will be referred to as a “MRFdamper” hereinafter).

Although the MRF is a liquid similar to a general hydraulic working oiland behaves as a Newtonian fluid when no magnetic filed is applied tothe MRF, the magnetic particles dispersed homogeneously within the MRFconcatenate along a direction of a magnetic filed and form chain-likeclusters when the magnetic field is applied from the outside. Becausethe clusters resist against deformation (flow), an apparent viscosity ofthe MRF sharply increases and the MRF behaves like a plastic fluidhaving yield stress when it flows. Such change of viscosity caused bythe magnetic field of the MRF is reversible. It is also possible tocontrol the degree of the viscosity of the MRF by controlling intensityof the magnetic field. This change in state of the MRF occurs veryquickly and its response to the change of the magnetic field is in orderof several milliseconds.

Generally, the MRF damper having the MRF is constructed as follows. Thatis, a piston partitions an inside of a cylinder tube filled with the MRFinto first and second oil chambers (first and second chambers). Thepiston is connected with a piston rod that projects out of one end ofthe cylinder tube. A rod guide that is disposed so as to close one endof the cylinder tube slidably supports the piston rod.

The piston is provided with a communication hole that circulates the MRFbetween the first and second oil chambers. The piston also contains anelectromagnetic coil for applying a magnetic field to the MRF within thecommunication hole. The viscosity of the MRF may be changed and avariable damping force may be obtained by controlling magnitude of themagnetic field applied to the MRF within the communication hole by powerfed to the electromagnetic coil.

The piston slides relative to the cylinder tube during when the MRFdamper operates, so that the magnetic particles within the MRF collideagainst an inner peripheral surface of the cylinder tube and an outerperipheral surface of the piston, causing a rubbing action. Because ironpowder and other is used as the magnetic particles, the rubbing actionlargely affects the cylinder tube and the piston if they are made ofmaterials whose hardness is smaller than iron. That is, sliding surfacesof the cylinder tube and the piston are largely abraded and it becomesdifficult to generate the damping force.

Then, U.S. Pat. No. 6,464,051 has proposed a configuration of forming aNi plating film on an inner peripheral surface of the cylinder tube intowhich the MRF is charged by implementing an electroless nickel (Ni)plating at first and then forming a Cr plating film by plating chrome(Cr) on the surface of the Ni plating film. While Vickers hardness ofthe Ni plating film formed by means of the electroless Ni plating isabout 550 to 700 VHN (Vickers Hardness Number), Vickers hardness of theCr plating film formed by means of Cr plating is about 900 to 1000 VHN.Thus, U.S. Pat. No. 6,464,051 suppresses the abrasion of the cylindertube otherwise caused by the magnetic particles by providing the Crplating film whose hardness is large on the sliding surface of thecylinder tube.

When the MRF damper is driven, not only the piston slides relative tothe cylinder tube but also its piston rod slides relative to the rodguide. Therefore, a technology for keeping sliding resistance (friction)between the piston rod and the rod guide low and for suppressingabrasion of the both sliding surfaces of the piston rod and the rodguide is required.

Then, JPA2000-514161 has disclosed a configuration in which a metal bushthat functions as a sealing member and a lubricant member is disposedbetween the piston rod and the rod guide. More specifically, the metalbush having a structure in which a porous layer made of bronze isprovided on an inner peripheral surface of a base material made ofnon-magnetic metal and in which fluorine resin is immersed in the porouslayer is used to slide the fluorine resin immersed layer relative to thepiston rod. Note that it is necessary to construct the MRF damper sothat no magnetic field acts on the MRF in parts other than thecommunication hole in order to obtain the variable damping-force bychanging the viscosity of the MRF within the communication hole providedin the piston. Therefore, the non-magnetic metal is used as the basematerial of the metal bush.

Still more, surface roughness of the piston rod is important in the MRFdamper from aspects of damping performance and durability of the damper.That is, while the sealing member is provided on the inner peripheralsurface of the cylinder tube in order to prevent the magnetic particleswithin the MRF from flowing out when the piston rod slides in the MRFdamper, there is a possibility that the magnetic particles enterirregular portions of the surface of the piston rod and flow out passingthrough the sealing member when the surface roughness of the piston rodis coarse (i.e., the surface roughness is large).

Then, as a technology of solving such problem, U.S. Pat. No. 6,516,926has disclosed a technology of providing a Cr plating film on an outerperipheral surface of the piston rod and of polishing and smoothing thesurface as a perfectly circular surface by using a tape polishing methodor the like.

The cylinder tube contacts with the piston in the MRF damper in a mannerof metal-to-metal contact in general and a clearance between thecylinder tube and the piston is required to be very accurate. However,because the Cr plating is electroplating (electrolytic plating), it isdifficult to uniformize a thickness of the Cr plating film bycontrolling Cr plating conditions. The thickness tends to vary even morein forming the Cr plating film on the inner peripheral surface of thecylinder tube. Therefore, the Cr plating film is formed into a thicknessfully larger than a thickness that is originally required for the Crplating and then the Cr plating film is ground and polished to adesirable thickness by way of honing or the like to meet with therequired precision of the abovementioned clearance.

When a cylinder tube having a Cr plating film 40 μm thick on its innerperipheral surface is to be manufactured for example, the Cr platingfilm is formed by Cr plating so that its maximum thickness becomes about100 μm on the inner peripheral surface of the cylinder tube and then thethickness is adjusted to 40 μm by grinding and polishing such as honing.However, such manufacturing method has problems that productivity dropsbecause it requires a long treatment time for the Cr plating and acertain processing time for grinding and polishing, e.g., honing, thefilm and that it is costly due to the costs required for the Cr platingand for grinding and polishing, e.g., honing, the film.

In a case of forming a Cr plating film on the piston rod, it is alsorequired to perform a rounding process while reducing surface roughnessby polishing the Cr plating film by using a polishing tape or the likeafter forming the Cr plating film that is fully thicker than a desiredthickness by Cr plating in the same manner with the case of the cylindertube. Accordingly, such manufacturing method of the piston rod hasproblems that it is costly due to polishing and that productivity dropsbecause it requires a long polishing time.

Still more, a circumstance in which the MRF damper is driven while beingbiased by a side force for example occurs frequently in a vehicle or thelike using the MRF damper. Under such circumstance, there is a problemin terms of durability that abrasion is accelerated because the fluorineresin impregnated layer of the metal bush is soft. Still more, becausethe magnetic particles contained in the MRF are very small, the magneticparticles infiltrate into the gap between the piston rod and the rodguide when the fluorine resin impregnated layer of the metal bushabrades away and accelerate the abrasion of the fluorine resinimpregnated layer further.

Then, as a method for solving such problems, it is conceivable to use amaterial obtained by impregnating fluorine resin such astetrafluoroethylene to hard alumite (referred to as “fluorine resincontained hard alumite” hereinafter) on the sliding surface of the rodguide. The fluorine resin contained hard alumite excels in abrasionresistance because it is harder than the fluorine resin impregnatedlayer of the metal bush and has characteristics that its slidingresistance is small as compared to the hard alumite because it containsthe fluorine resin.

However, the inventors found that the fluorine resin contained hardalumite has a problem that it indicates a relatively large slidingresistance value in a state being pressed by a large force. Therefore,the MRF damper structured by using the rod guide having the fluorineresin contained hard alumite on its sliding surface that slides relativeto the piston rod has a possibility of dropping accuracy in outputting atarget damping force if the MRF damper is biased by the side force orthe like when a driving signal (specifically, this indicates magnitudeof an electric current flown to the electromagnetic coil and is referredto as an “input signal” hereinafter) is inputted to the MRF damper toobtain a desirable damping force because the sliding resistance betweenthe piston rod and the rod guide increases. Still more, the increase ofthe sliding resistance between the piston rod and the rod guide tends toadvance the abrasion of the sliding surfaces of the piston rod and therod guide. It further causes various problems that drop the durabilityof the damper by causing a rickety piston rod supporting state, a leakof the MRF and the like.

The present invention has been made in view of the problems describedabove and seeks to provide a variable damping-force damper that iscapable of suppressing the drop of operational accuracy in the statebeing biased by the side force or the like and has excellent durabilityas well as to provide a manufacturing method of the same with highproductivity and at low cost.

SUMMARY OF THE INVENTION

A variable damping-force damper of the invention includes:

a cylinder tube filled with a working fluid that is a magnetic fluid ora magneto-rheological fluid containing magnetic particles; and

a piston that partitions an inside of the cylinder tube into first andsecond chambers, has a communication hole for circulating the work fluidbetween the first and second chambers and has an electromagnetic coilfor applying a magnetic field to the work fluid within the communicationhole;

the variable damping-force damper controlling a damping force bychanging viscosity of the work fluid within the communication hole byfeeding power to the electromagnetic coil;

wherein the variable damping-force damper has a sliding surface thatslides under an influence of the magnetic particles during itstelescopic motion; and

the sliding surface has a nickel plating film whose Vickers hardness is800 VHN or more on the surface thereof.

Such Ni plating film whose Vickers hardness is 800 VHN or more may beformed by implementing electroless nickel plating on the sliding surfaceand by treating the Ni plating film formed by the electroless nickelplating by heat for 0.5 to 5 hours at 200° C. to 600° C.

According to the variable damping-force damper of the invention, thehardness of the Ni plating film formed on the sliding surface isequalized with that of the Cr plating film of the prior art, so that theNi plating film shows excellent durability. Still more, because it ispossible to form the Ni plating film having a desirable precisethickness (measurement precision) in an electroless Ni plating step, itis not necessary to carry out grinding and polishing such as honing.Accordingly, it is also possible to obtain high productivity.

According to the variable damping-force damper of the invention,preferably the Ni plating film contains phosphorus as well as one orplurality of elements selected from boron, tungsten, boron nitride andsilicon carbide. The phosphorus, boron and tungsten are contained in theNi plating film by chemically bonding with nickel and the boron nitrideand silicon carbide are dispersed and contained in the Ni plating film.The boron, tungsten, boron nitride and silicon carbide may be containedin the Ni plating film by codepositing respectively with nickel.

By constructing as described above, it becomes possible to reduce acoefficient of friction of the Ni plating film and to adjust to adesirable coefficient of friction. Thereby, it becomes possible toadjust abrasion resistance of the Ni plating film itself and to controlits aggression against a sliding mating material. Still more, it becomespossible to obtain excellent durability (abrasion resistance) byhardening the Ni plating film.

In the variable damping-force damper of the invention, the slidingsurface is an outer peripheral surface of the piston or an innerperipheral surface of the cylinder tube and preferably, the Ni platingfilm is provided at least on the inner peripheral surface of thecylinder tube. In this case, preferably, a thickness of the Ni platingfilm is 15 μm or more when a length of sliding portion of the piston inan axial core direction is 50 mm and when the length of the slidingportion is shorter than 50 mm, the thickness of the Ni plating film isset such that the shorter the length of the sliding portion, the thickerthe thickness of the Ni plating film becomes beyond 15 μm.

It becomes possible to arrange so that elapsed abrasion of the Niplating film formed on the inner peripheral surface of the cylinder tubedoes not depend on the length of the sliding portion of the piston thatslides relative to the Ni plating film in the axial core direction byconstructing as described above.

The variable damping-force damper of the invention further includes apiston rod whose one end is attached to the piston and whose other endextends to the outside of the cylinder tube. The cylinder tube has acylindrical rod guide section disposed at one end thereof so that thepiston rod is inserted through the rod guide. In this case, the slidingsurface is an interfacial sliding surface of the inner peripheralsurface of the rod guide and the outer peripheral surface of the pistonrod and the Ni plating film is preferably provided at least on the outerperipheral surface of the piston rod.

Because it is possible not only to suppress the abrasion of the pistonrod but also to reduce the surface roughness of the outer peripheralsurface of the piston rod by forming the Ni plating film by means ofelectroless nickel plating on the outer peripheral surface of the pistonrod as described above, it is also possible to prevent the magneticparticles from otherwise leaking out of the cylinder tube due to theirregularity of the outer peripheral surface of the piston rod and toimprove the durability thereof.

The variable damping-force damper of the invention includes a cylindertube in which a working fluid containing magnetic particles is filled, apiston slidably disposed within the cylinder tube, a piston rod disposedsuch that one end thereof is attached to the piston and another endprotrudes out of one end of the cylinder tube and a rod guide thatcloses one end of the cylinder tube and slidably supports the pistonrod, wherein the rod guide has a heat-treated electroless Ni platingfilm containing phosphorus and fluorite resin on the sliding surfacethat slides relative to the piston rod.

The rod guide is manufactured through a process including an electrolessnickel plating step of forming the electroless Ni plating filmcontaining phosphorus and fluorine resin on a surface that slidesrelative to the piston rod in the predetermined base material portioncomposing the rod guide and a heat treatment step of implementing a heattreatment on the electroless Ni plating film formed by the electrolessnickel plating step. It is noted that the electroless Ni plating filmcontaining the phosphorus and fluorine resin may be formed by using aplating solution in which the fluorine resin is doped in an electrolessNi plating solution utilizing a reducing effect of hypophosphorus andthe phosphorus and fluorine resin codeposit in precipitating nickel.

The variable damping-force damper of the invention reduces slidingresistance between the piston rod and the rod guide by reducing thecoefficient of friction of the sliding surface of the rod guide by theelectroless Ni plating film containing the fluorine resin, so that itbecomes possible to reduce a drop of the operational accuracy of thedamper with respect to an input signal even in a state biased by a sideforce or the like.

Preferably, the Vickers hardness of the electroless Ni plating filmplated on the rod guide is 360 VHN or more in the variable damping-forcedamper of the invention. In other words, a treatment condition of theheat treatment step of the electroless Ni plating film is set such thatthe Vickers hardness of the electroless Ni plating film becomes 360 VHNor more. It allows the abrasion resistance of the rod guide to beimproved.

Preferably, the variable damping-force damper of the invention isarranged such that the rod guide has a base material portion made ofaluminum or aluminum alloy (referred to as the “Aluminum alloy or thelike” hereinafter), the electroless Ni plating film is formed on apredetermined surface of the base material portion and the piston rodslides relative to the electroless Ni plating film.

When the variable damping-force damper is driven while being biased bythe side force or the like, its stress may be relaxed because the rodguide (base material portion) that receives the stress from the pistonrod slightly elastically deforms when the aluminum alloy or the like isused for the base material portion of the rod guide as described above.It is also possible to suppress an increase of the sliding resistancebetween the piston rod and the rod guide because a face-to-face contactstate of the piston rod and the rod guide is maintained. Thus, itbecomes possible to keep the drop of the operational accuracy of thevariable damping-force damper with respect to an input signal low and tosuppress frictional abrasion of the both sliding surfaces of the pistonrod and the rod guide (specifically, occurrence of biased abrasioncaused by locally increased friction). Note that because the aluminumalloy or the like is inexpensive and excels in workability, it hasmerits that the rod guide that excels in accuracy of form may bemanufactured at low cost with high productivity.

Preferably, the aluminum alloy is used for the base material portion ofthe rod guide in the variable damping-force damper of the invention andin that case, preferably the base material portion is formed in a statein which the alloy ingredients other than Al (referred to as “alloyingredients” hereinafter) precipitate. It becomes easy to obtainwell-balanced mechanical characteristics such as hardness, yield point,tensile strength and others desired as a structural part by using thealuminum alloy for the base material portion of the rod guide. When thebase material portion is in the state in which the alloy ingredientsprecipitate, it becomes possible to suppress various metal atomscomposing the aluminum alloy from moving due to dislocation and to keepthe mechanical characteristics. Meanwhile because the electroless nickelplating film is treated by heat so that it has the predetermined Vickershardness as described above, it is preferable to balance the compositionand others of the aluminum alloy with the heat treatment conditions sothat the precipitation state of the alloy ingredients in the aluminumalloy does not change as much as possible by this heat treatment. Stillmore, it is preferable to arrange so that the alloy ingredientsprecipitated in the aluminum alloy do not fall into a state in whichthey become solid-soluble to Al by the heat treatment step of theelectroless Ni plating film.

In the variable damping-force damper of the invention, preferably theelectroless Ni plating film has a two-layered structure of a firstelectroless Ni plating layer formed on the surface of the base materialportion and containing phosphorus and of a second electroless Ni platinglayer formed on the surface of the first electroless Ni plating layerand containing phosphorus and fluorine resin and preferably, the firstelectroless Ni plating layer has hardness that is harder than that ofthe second electroless Ni plating layer.

It becomes possible to reduce the sliding resistance between the pistonrod and the rod guide because the second electroless nickel platinglayer slides relative to the rod guide in an initial period of usage ofthe variable damping-force damper. When the second electroless nickelplating layer abrades with time, the sliding resistance decreasesbecause the clearance between the piston rod and the rod guide iswidened. At this time, the first electroless nickel plating layer thathas the hardness harder than the second electroless nickel plating layerslides relative to the rod guide, so that the first plating layer cansuppress the advance of abrasion of the first electroless nickel platinglayer. Thus, it becomes possible to keep the excellent durability.

Preferably, the rod guide is provided with a sealing member to preventthe working fluid from leaking out of the cylinder tube in the variabledamping-force damper of the invention. It is also preferable to providethe sealing member on the side closer to the piston rather than theelectroless Ni plating film in the axial core direction of the cylindertube. Thereby, the sealing member can reduce a number of the magneticparticles entering the sliding surface between the rod guide and thepiston rod and can suppress the abrasion of the both sliding surfaces ofthe rod guide and the piston rod.

Preferably, the piston rod is provided with an electroless Ni platingfilm or a Cr plating film on the sliding surface thereof with athickness of 10 μm or more and a surface roughness of 0.1 to 1.5 interms of a Rz value. It becomes possible to reduce the magneticparticles entering the gap between the piston rod and the rod guide andto suppress the abrasion of the sliding surface by smoothing the surfaceof the piston rod.

As described above, according to the present invention, it is possibleto obtain the excellent abrasion resistance and to reduce the abrasionby forming the highly hard Ni plating film on the sliding surface thatslides under influence of the magnetic particles. Thereby, it becomespossible to improve the durability of the variable damping-force damperand to keep its damping for constant for a long period of time. Stillmore, because the plating process that is implemented on thepredetermined sliding surface may be accomplished only by theelectroless nickel plating, it becomes possible to simplify the platingprocess and to reduce costs required for the plating process. Further,because the Ni plating film having a desirable precision may be formedonly by the electroless nickel plating, it becomes unnecessary to carryout grinding and polishing such as honing to adjust dimensions when theNi plating film is formed on the cylinder tube for example. Thus, theproduction cost may be lowered further. It is also possible to suppressthe magnetic particles from being discharged out of the cylinder tubeand to improve the durability by forming the Ni plating film on theouter peripheral surface of the piston rod to reduce its surfaceroughness.

The variable damping-force damper of the invention is also provided withthe electroless Ni plating film in which fluorine resin is codepositedand whose coefficient of friction is small on the sliding surface of therod guide that slides relative to the piston rod, so that it is possibleto suppress an increase of the sliding resistance between the rod guideand the piston rod even when the piston rod is biased by the side forceor the like and hence to suppress the abrasion of the both slidingsurfaces of the rod guide and the piston rod. Still more, the use of thealuminum alloy or the like for the base material portion of the rodguide allows to suppress an increase of the sliding resistance betweenthe rod guide and the piston rod when the piston rod presses the rodguide due to the biased force such as the side force because thealuminum alloy or the like relaxes the stress by its elasticdeformation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a section view showing a schematic structure of a variabledamping-force damper according to a first embodiment of the invention;

FIG. 2 is a partially enlarged section view of a piston composing thevariable damping-force damper of the invention;

FIG. 3 is a section view diagrammatically showing a method for carryingout a durability test;

FIG. 4 diagrammatically shows a method for measuring abrasion caused inthe durability test;

FIG. 5 is a perspective view diagrammatically showing a state of contactof a cylindrical member with a member having a concave curved surface;

FIG. 6A is a perpendicular partial section view including an axial coreof a piston rod diagrammatically showing a structure of the piston rodin a rod guide composing the variable damping-force damper of theembodiment and a sliding state of a rod guide section and the pistonrod;

FIG. 6B is a section view of the rod guide composing the variabledamping-force damper of the embodiment taken along A-A in FIG. 6A;

FIG. 6C is a section view of a reference example diagrammaticallyshowing the structure of the piston rod and the sliding state of the rodguide section and the piston rod;

FIG. 7 is a flowchart showing a manufacturing method of the rod guide;

FIG. 8A is a perpendicular section view including an axial core of apiston rod showing a structure of a first modified example of the rodguide composing the variable damping-force damper of the embodiment;

FIG. 8B is a section view showing the first modified example of the rodguide composing the variable damping-force damper of the embodimenttaken along A-A in FIG. 8A;

FIG. 9A is a section view of the rod guide including the axial core ofthe piston rod diagrammatically showing a state in which the piston rodis pressed by side force locally against the rod guide whose basematerial portion has large Young's modulus relatively in a radialdirection thereof;

FIG. 9B is a section view of the rod guide including the axial core ofthe piston rod diagrammatically showing a state in which the piston rodis pressed by side force locally against the rod guide whose basematerial portion has small Young's modulus relatively in a radialdirection thereof;

FIG. 10A is a perpendicular section view including an axial core of apiston rod showing a structure of a second modified example of the rodguide composing the variable damping-force damper of the embodiment;

FIG. 10B is a section view showing the second modified example of therod guide composing the variable damping-force damper of the embodimenttaken along A-A in FIG. 10A;

FIG. 11 is a graph showing changes of an inner diameter of a cylindertube in an axial core direction after a durability test of the variabledamping-force damper of the first embodiment;

FIG. 12 is a graph showing a relationship between abrasion Δt of a Niplating film and surface pressure PMEAN in the variable damping-forcedamper of the first embodiment;

FIG. 13 is a graph showing a relationship between the abrasion Δt of theNi plating film and a length of a piston sliding portion in the variabledamping-force damper of the first embodiment;

FIG. 14 is a graph showing total abrasion of a piston ring and the Niplating film in each variable damping-force damper of the firstembodiment and a comparative example;

FIG. 15 is a graph showing magnitude of friction between the cylindertube and the piston in each variable damping-force damper of the firstand second embodiments;

FIG. 16 is a section view of the variable damping-force damperschematically showing a testing method for measuring friction betweenthe piston rod and the rod guide; and

FIG. 17 is a graph showing measured test results of the frictions.

BEST MODE FOR CARRYING OUT THE INVENTION

A mode for carrying out the invention will be explained in detail belowwith reference to the drawings. Firstly, an overall structure of avariable damping-force damper will be explained.

FIG. 1 is a section view showing a schematic structure of the variabledamping-force damper according to one embodiment of the invention.

The variable damping-force damper 10 has a so-called mono-tube(de-carvone type) structure and has a cylindrical cylinder tube 12filled with MRF (magnetic fluid or magneto-rheological fluid) in whichmagnetic particles are dispersed in oil and the like, a piston rod 13slidable in an axial core direction (longitudinal direction) of thecylinder tube 12, a piston 16 attached at an edge of the piston rod 13to partition an inside of the cylinder tube 12 into a first oil chamber(first chamber) 14 and a second oil chamber (second chamber) 15 and afree piston 18 that partitions a high pressure gas chamber 17 from thesecond oil chamber 15.

A rod guide 19 is provided at one end of the cylinder tube 12 to closean opening of the cylinder tube 12. Substantially, the rod guide 19 hasa cylindrical shape and supports the piston rod 13 that is insertedthrough a center hole of the rod guide 19. Then, an outer peripheralsurface of the piston rod 13 (a sliding surface of the piston rod 13)slides with an inner peripheral surface of the rod guide 19 (a slidingsurface of the rod guide 19). The rod guide 19 is also provided with apacking 26 for preventing the MRF from leaking to an outside of thepiston. The structures of the piston rod 13 and the rod guide 19 will bedetailed later, respectively.

An eyepiece 12 a is provided at the other end of the cylinder tube 12.When the variable damping-force damper 10 is used as a suspension of avehicle for example, a bolt not shown is inserted through the cylindertube 12 a and is linked with a trailing arm, a wheel-side member.Another end (not shown) of the piston rod 13 is linked with a damperbase (upper part of a wheel house) that is a body-side member. Vibrationtransmitted from the wheel side to the body side during traveling isdamped by the cylinder in which the outer peripheral surfaces of thepiston 16 and the free piston 18 slide relative to the inner peripheralsurface of the cylinder tube 12.

The piston 16 has a communication hole 35 that communicates the firstoil chamber 14 with the second oil chamber 15 and an electromagneticcoil 52 that applies a magnetic field to the MRF within thecommunication hole 35. Electric current is supplied to the magnetic coil52 by using a 53 connected with the magnetic coil 52. The power feedingline 53 is taken out of the outside (its state is not shown) of thecylinder through an inside of the piston rod 13 and is connected with apredetermined control power source not shown. When the electric currentis supplied from the control power source to the magnetic coil 52through the power feeding line 53, a magnetic field is applied to theMRF circulating through the communication hole 35, the magneticparticles contained in the MRF form chain clusters and apparentviscosity of the MRF passing through the communication hole 35increases. Thus, it is possible to control damping force variably bycontrolling magnitude of the magnetic field applied to the MRF.

Next, the structure of the piston 16 will be explained.

FIG. 2 is a partially enlarged section view of the piston composing thevariable damping-force damper. The piston 16 has a piston core 38 thatfits with the piston rod 13, side covers 39 a and 39 b providedrespectively at ends of the piston core 38 in an axial direction, themagnetic coil 52 embedded in the piston core 38 near an outercircumference of the piston core 38 and a cylindrical piston ring 34surrounding the piston core 38 so that a certain gap 35 c is formedbetween the piston ring 34 and the outer circumference of the pistoncore 38.

The side covers 39 a and 39 b are provided with holes 35 a and 35 b andthe communication hole 35 is composed of the holes 35 a and 35 b and thegap 35 c that communicate from each other in the piston 16. The firstoil chamber 14 communicates with the second oil chamber 15 through thecommunication hole 35 and the MRF circulates through the communicationhole 35. Specifically, when the electric current is supplied from thecontrol power source (not shown) to the magnetic coil 52 through thepower feeding line 53, the magnetic field is applied to the MRFcirculating through the communication hole 35 c, the ferromagneticparticles contained in the MRF form chain clusters and the apparentviscosity of the MRF passing through the communication hole 35 cincreases. Thus, it is possible to control the damping force variably bycontrolling the magnitude of the magnetic field applied to the MRF asdescribed above.

The piston ring 34 has a cylindrical shape and its both ends are tightlysealed to the side covers 39 a and 39 b by way of caulking and the like.As shown in FIG. 2, an outer diameter of the piston ring 34 is notconstant along the axial direction. That is, although the outer diameteris constant within a certain length range at a center part of the ringalong the axial direction, the outer diameter becomes short as it goesto the end. The portion where the outer diameter is constant is asliding portion that substantially slides relative to the innerperipheral surface of the cylinder tube 12. A length of the slidingportion in the axial direction is denoted as “L” and will be referred toas a “length of the piston sliding portion L” hereinafter. It is notedthat the length of piston sliding portion L is a parameter related to athickness of a Ni plating film 22 (described later) formed on the innerperipheral surface of the cylinder tube 12 and this relationship will bealso detailed later.

Next, the structure of the cylinder tube 12 and a manufacturing methodof the same will be explained. As shown in FIG. 2, the cylinder tube 12has a structure in which the Ni plating film 22 is formed on an innerperipheral surface of a cylinder base material 21 made from metal suchas iron and stainless steel. The piston 16 and the free piston 18 (seeFIG. 1) slide relative to the Ni plating film 22. The Ni plating film 22is crystal and its Vickers hardness is 800 VHN or more. Because thehighly hard Ni plating film 22 has thus excellent abrasion resistance,it is possible to suppress elapsed abrasion of the inner peripheralsurface of the cylinder tube 12 and to maintain a damping force of thevariable damping-force damper 10 at constant for a long period of time.

It is possible to form the Ni plating film 22 by applying electroless Niplating on the inner peripheral surface of the cylinder tube 12 and bytreating the Ni plating film thus formed by heat at 200° C. to 600° C.for 0.5 to 5 hours. It is noted that the Ni plating film 22 may beformed also on the outer peripheral surface of the cylinder tube 12.Although the Ni plating film (before the heat treatment) formed by theelectroless Ni plating is amorphous, the Ni plating film is crystallizedby the heat treatment. It happens because although phosphorus (P) isgenerally contained in the Ni plating film (before the heat treatment)formed by the electroless Ni plating due to a process of a chemicalreaction of the electroless Ni plating, the heat treatment causes achemical reaction of Ni and P, forming a crystal phase of Ni₃P.

Because it is necessary to contact the cylinder tube 12 and the piston16 in a manner of metal-to-metal contact in the variable damping-forcedamper 10, a high precision, i.e., specifically a value around 50 μm, isrequired for a difference between an inner radium of the cylinder tube12 and an outer radius of the piston 16 at the sliding portion, i.e.,for a clearance between the cylinder tube 12 and the piston 16 (referredto simply as a “clearance” hereinafter).

It is possible to grow the Ni plating film 22 to a desirable thicknesswhile readily controlling an uniformity of thickness of the Ni platingfilm 22 in a level of ±2 to 3 μm by the electroless Ni plating. Becausethe precision of the uniformity of the thickness required to the Niplating film 22 can be realized in the process of the electroless Niplating, it is possible to realize a precision required also to theclearance. Accordingly, it is possible to reduce a machining cost andthus a production cost because it is not necessary to adjust thethickness of the Ni plating film 22 by conducting grinding and polishingprocesses such as honing in a manufacturing process of the cylinder tube12 or of the variable damping-force damper 10. Still more, it ispossible to cut a cost regarding the plating process because Cr platingthat has been required in the past is not necessary and the platingprocess may be simplified. It is noted that grinding and polishing suchas honing may be carried out on the Ni plating film 22.

Next, composition and frictional characteristics of the Ni plating film22 will be explained. Preferably, the Ni plating film 22 containsphosphorus (P) as described above. P may be codeposited with Ni by usinghypophosphorus (H₃PO₂) that is a general reducing agent used inelectroless Ni plating. Preferably, the Ni plating film 22 furthercontains one or plurality of elements selected from boron (B), tungsten(W), boron nitride (BN) and silicon carbide (SiC) (referred to as“additional ingredients” hereinafter). B and W exist within the Niplating film 22 with a form chemically bonded with Ni (described as“Ni—P—W” and “Ni—P—B” hereinafter). Meanwhile, BN and SiC are dispersedwithin the Ni plating film 22 and compose a metal-ceramics compoundmaterial (described as “Ni—P+BN” and “Ni—P+SiC” hereinafter). B, W, BNand SiC may be contained respectively in the Ni plating film 22 byeutectoid from a plating solution.

It is noted that the additional ingredients to the Ni plating film 22are not limited the various elements (metals) and chemical compoundsdescribed above. It is also possible to use boron carbide (B₄C), siliconnitride (Si₃N₄), alumina (Al₂O₃), zirconia (ZrO₂), aluminum nitride(AIN), diamond (C) or the like for example.

As concrete examples, Table 1 shows characteristics of a Ni plating filmcontaining no those additional ingredients and Ni plating films 22containing respectively W, BN and SiC together with the Cr plating film(prior art example) with respect to Vickers hardness and frictionalcharacteristics. Although the Ni—P plating film containing no additionalingredient has characteristics that its coefficient of friction islarger than that of the Cr plating film, it has a merit that its mateaggression is low because its Vickers hardness is small as shown inresults of a durability test described later even though Table 1 doesnot show that.

TABLE 1 CLASSIFICATION PRIOR ART PRESENT INVENTION PLATINGELECTRO-PLATING ELECTROLESS-PLATING PLATING FILM HARD Cr Ni—P Ni—P—WNi—P; BN Ni—P + SiC VICKERS' HARDNESS 900 to 1000 800 to 900 900 to 10001000 to 1100 1300 to 1400 COEFFICIENT OF 0.6 0.8 0.6 0.6 0.72 FRICTION(COEFFICIENT OF DYNAMIC FRICTION) PIN ABRASION (mg) 2.79 — 0.87 0/06 3.5(AGGRESSION AGAINST SLIDING MEMBER) NOTE: The coefficient of friction isa value with respect to SUJ steel (bearing steel) and the PIN abrasionis that of the SUJ steel.

It is possible to adjust the coefficient of friction of the Ni platingfilm 22 to a desirable value within a certain range depending onmaterial properties of the additional ingredients described above byappropriately selecting the additional ingredients and controlling anadded amount. As shown in Table 1, the coefficient of friction of therespective plating films of Ni—P—W, Ni—P+BN and Ni—P+SiC is smaller thanthat of the Ni—P plating film. It can be seen from this fact that W, BNand SiC have an effect of reducing the coefficient of friction of the Niplating film 22. The reduction of the coefficient of friction of the Niplating film 22 allows the aggression of the Ni plating film 22 ageistthe sliding surface of the piston 16 (the sliding surface of the pistonring 34) to be lowered and thereby the abrasion of the sliding surfaceof the piston 16 to be reduced.

The Vickers hardness of the Ni plating film 22 may be adjusted to adesirable value within a certain range depending on material propertiesof the additional ingredients described above by appropriately selectingthe additional ingredients and controlling their added amount. TheVickers hardness of the respective plating films of Ni—P—W, Ni—P+BN andNi—P+SiC is larger than that of the Ni—P plating film as shown inTable 1. That is, it can be seen from the above that W, BN and SiC havean effect of increasing the Vickers hardness of the Ni plating film 22.Such effect may be obtained in the same manner also when B, B₄C andSi₃N₄ are used as additional ingredients. It becomes possible to improveabrasion resistance of the Ni plating film 22 itself by enhancing theVickers hardness of the Ni plating film 22. In contrary to that, it ishard to form a Cr plating film whose Vickers hardness exceeds 1000 VHNby Cr plating.

As shown in Table 1, although the Ni—P—W plating film hascharacteristics that its Vickers hardness and coefficient of frictionare equal to those of the Cr plating film, it has merits that its PINabrasion is small and mate aggression is low. Although the Ni—P—Bplating film has characteristics that its Vickers hardness is slightlylarger than that of the Cr plating film and its coefficient of frictionis equal to that of the Cr plating film, its PIN abrasion is small andmate aggression is low. In the case of the Ni—P—SiC plating film, itsVickers hardness larger than that of the Cr plating film and itscoefficient of friction is slightly larger than that of the Cr platingfilm. It can be seen that the PIN abrasion of the Ni—P—SiC plating filmis larger than that of the Cr plating film and that its mate aggressionis enhanced. The Ni—P—SiC plating film is used not from an aspect oflowering the mate aggression but from an aspect of that the Ni—P—SiCplating film itself shows excellent abrasion resistance. When theNi—P—SiC plating film is used as the Ni plating film 22, it ispreferable to select a material whose abrasion becomes small as thepiston ring 34 when it slides relative to the Ni—P—SiC plating film.

It is preferable to adjust the coefficient of friction and the Vickershardness of the Ni plating film 22 by considering the material propertyof the piston ring 34 so that frictional balance between the cylindertube 12 and the piston 16 is improved, e.g., so that total abrasion ofthe sliding portions of the Ni plating film 22 and the piston 16 isminimized. Thereby, it becomes possible to obtain the variabledamping-force damper 10 having excellent durability. Still more, itbecomes possible to keep the damping force of the variable damping-forcedamper 10 constant for a long period of time by reducing the totalabrasion of the sliding portions of the Ni plating film 22 and thepiston 16.

Next, implications between the thickness of the Ni plating film 22 andthe length of the piston sliding portion L of the piston will beexplained.

The thickness of the Ni plating film 22 is a factor that determines thedurability (life) of the variable damping-force damper 10. Thedurability improves if the thickness of the Ni plating film 22 isincreased. However, if the thickness of the Ni plating film 22 isincreased more than what is required, it drops productivity andincreases a production cost. Therefore, it is preferable toappropriately determine the thickness of the Ni plating film 22 (leastrequired thickness) considered as adequate from aspects of durabilityand productivity by considering use environment.

The thickness of the Ni plating film 22 is preferable to be 15 μm ormore when a length of the piston sliding portion L with respect to theNi plating film 22 is 50 mm in the variable damping-force damper 10.When the length of the piston sliding portion L is shorter than 50 mm,it is preferable to set the thickness of the Ni plating film 22 so thatthe shorter the length of the piston sliding portion L, the thicker thethickness of the Ni plating film 22 becomes beyond 15 μm. The variabledamping-force damper 10 described above may be suitably used for asuspension of a vehicle.

Note that “the shorter the length of the piston sliding portion L, thethicker the thickness of the Ni plating film 22 becomes beyond 15 μm”described above in setting the thickness of the Ni plating film 22indicates specifically that “the thickness of the Ni plating film 22 isincreased so that it becomes inversely proportional to a square root ofthe length of the piston sliding portion L.” A reason why the length ofthe piston sliding portion L is set as 50 mm and the thickness of the Niplating film 22 as 15 μm as reference values will be explained inembodiments described later.

If the thickness of the Ni plating film 22 is under 15 μm when thelength of the piston sliding portion L is 50 mm, a time until when thebase of the cylinder tube 12 appears due to the abrasion of the Niplating film 22 becomes short and enough durability cannot be obtained.An upper limit value of the thickness of the Ni plating film 22 may bedefined in terms of a safety factor, productivity and a production costand is from 20 μm to 30 μm.

The thickness of the Ni plating film 22 is changed in accordance to thelength of the piston sliding portion L because the thickness requiredfor the Ni plating film 22 changes depending on a surface pressurebetween the sliding surface of the piston 16 and the Ni plating film 22(simply referred to as the “surface pressure” hereinafter). When anexternal force (side force) of predetermined magnitude acts in a radialdirection of the piston 16 for example and if the length of the pistonsliding portion L is short, a contact area of the piston 16 and the Niplating film 22 decreases and the surface pressure increases. Theshorter the length of the piston sliding portion L, the harder africtional environment between the piston 16 and the cylinder tube 12becomes, increasing abrasion of the Ni plating film 22. Therefore, it ispreferable to increase the thickness of the Ni plating film 22 as thelength of the piston sliding portion L is shortened so that theplurality of kinds of variable damping-force dampers 10 having differentpiston sliding portion lengths L may have a constant product life.

A concrete relationship between The length of the piston sliding portionL and the thickness of the Ni plating film 22 when the length of thepiston sliding portion L is under 50 mm may be found based on abrasionof the Ni plating film 22 in a durability test using a piston 16 havinga predetermined length of the piston sliding portion L, a surfacepressure in the durability test and an elastic contact theory, e.g.,Hertz Elastic Contact Theory. This method will be briefly explainedbelow.

The following is an outline of the durability test carried out here.

FIG. 3 is a section view diagrammatically showing a method for carryingout the durability test and FIG. 4 diagrammatically shows a method formeasuring abrasion caused in the durability test. The durability test iscarried to find the abrasion (abraded thickness) Δt of the Ni platingfilm 22 by reciprocating the sliding portion of the piston 16 within acertain length range (referred to as a “piston sliding range”hereinafter) in the axial core direction in the cylinder tube 12 whileapplying a certain force (side force) F in the radial direction (in thevertically lower direction here) of the piston 16 and by measuringchanges of the inner diameter of the cylinder tube 12 as shown in FIG.3.

Before starting the durability test, dependency of the inner diameter inthe axial core direction remains within a tolerance and it can be saidthat there is substantially no such dependency. However, after endingthe durability test, biased abrasion occurs on the inner peripheralsurface of the cylinder tube 12 by the side force F and the abrasion Δtdiffers depending on the radial direction. Then, in order to correlatethe abrasion Δt of the Ni plating film 22 with the direction of the sideforce F, the inner diameter of the cylinder tube 12 in the direction ofthe side force F is defined as a “F-direction inner diameter” and theinner diameters of the cylinder tube 12 in the radial direction crossingrespectively with the direction of the side force F at 45°, 90° and 135°are defined as “inner diameter in 45° direction,” “inner diameter in 90°direction” and “inner direction in 135° direction” as shown in FIG. 4 tomeasure the respective inner diameters at predetermined intervals in theaxial core direction by using a commercially available cylinder gaugeand the like. It enables one to find the abrasion Δt of the Ni platingfilm 22 in each direction. Note that FIG. 4 specifically shows theabrasions in the F-direction and in the 45° direction.

A method for calculating the relationship between the thickness of theNi plating film 22 and the length of the piston sliding portion L iscarried out as follows. FIG. 5 is a perspective view diagrammaticallyshowing a state of contact of a cylindrical member with a member havinga concave curved surface. The cylindrical member 41 shown in FIG. 5 isregarded as the piston ring 34 and the concave curved member 42 as theNi plating film 22, to find shapes and material properties of the pistonring 34 and the Ni plating film 22, a magnitude W of the side force Fand a surface pressure PMEAN between the piston ring 34 and the Niplating film 22.

Specifically, Young's modulus of the piston ring 34 having the length ofthe piston sliding portion L is denoted by E1, Poisson's ratio by v1 andan outer radius by R1 and Young's modulus of the Ni plating film 22 isdenoted by E2, Poisson's ratio by v2 and an outer radius by R2. When themagnitude (weight) of the side force F is W, equivalent Young's modulusE may be given by Eq. 1, an equivalent radius of curvature R may begiven by Eq. 2, a contact half-width b may be given by Eq. 3 and acontact area A may be given by Eq. 4, respectively, according to theHertz's elastic contact theory. Then, a maximum hertz pressure PMAX maybe given by Eq. 5 and a surface pressure (average hertz pressure) PMEANmay be given by Eq. 6.

It is possible to consider that the abrasion Δt of the Ni plating film22 is primarily correlated with the surface pressure PMEAN. Then, Eq. 7holds if its proportional constant is set as K. A relationship expressedby Eq. 8 holds between the abrasion Δt of the Ni plating film 22 and thelength of the piston sliding portion L from Eqs. 1 through 7 and it canbe seen that the abrasion Δt of the Ni plating film 22 is inverselyproportional to a square root of the length of the piston slidingportion L. The abrasion Δt of the Ni plating film 22 is also the leastminimum thickness required for the Ni plating film 22. Accordingly, theleast minimum thickness of the Ni plating film 22 to be set when thelength of the piston sliding portion L is changed may be found based onEq. 8. It is noted that the least minimum thickness required for the Niplating film 22 may be found by Eq. 8 also when the radius R1 of thepiston ring 34 or the radius R2 of the Ni plating film 22 is changed(these changes are reflected in R) and when the materials of the pistonring 34 and the Ni plating film 22 are changed (these changes arereflected in E).

$\begin{matrix}\text{[Equations]} & \; \\{\frac{1}{E} = {\frac{1}{2}\left\lbrack {\frac{1 - v_{1}^{2}}{E_{1}} + \frac{1 - v_{2}^{2}}{E_{2}}} \right\rbrack}} & {{Eq}.\mspace{14mu} 1} \\{\frac{1}{R} = {\frac{1}{R_{1}} - \frac{1}{R_{2}}}} & {{Eq}.\mspace{14mu} 2} \\{b = \sqrt{\frac{8{RW}}{\pi\;{EL}}}} & {{Eq}.\mspace{14mu} 3} \\{A = {2{bL}}} & {{Eq}.\mspace{14mu} 4} \\{P_{MAX} = {\frac{2}{\pi}\frac{W}{bL}}} & {{Eq}.\mspace{14mu} 5} \\{P_{MEAN} = {\frac{\pi}{4}P_{MAX}}} & {{Eq}.\mspace{14mu} 6} \\{{\Delta\; t} = {K \cdot P_{MEAN}}} & {{Eq}.\mspace{14mu} 7} \\{{\Delta\; t} = {\frac{K}{4}\sqrt{\frac{\pi\;{EW}}{2{RL}}}}} & {{Eq}.\mspace{14mu} 8}\end{matrix}$

A structure of the piston rod and the sliding portion with the rod guidesection of the variable damping-force damper of the present embodimentwill be explained below.

At first, the structure of the piston rod will be explained.

FIG. 6A is a perpendicular partial section view including the axial coreof the piston rod 13 diagrammatically showing the structure of thepiston rod 13 and the sliding portion between the rod guide section andthe piston rod. FIG. 6C shows a reference example of the piston rod.

As shown in FIG. 6A, the piston rod 13 has a structure in which a Niplating film 24 is formed on outer periphery of a rod base material 25by means of electroless Ni plating. In the variable damping-force damper10, a fluorine resin contained Ni plating film 32 containing fluorineresin described later formed on the rod guide 19 slides relative to theNi plating film 24 formed on the piston rod 13.

When surface roughness of an outer peripheral surface of the rod iscoarse as shown in FIG. 6C, small magnetic particles enter concaveportions around the rod. If the rod slides to an atmosphere side(outside) in this state, the magnetic particles entered the concaveportions are discharged to the atmosphere. That is, there is a problemthat the concave portions around the rod act as a pump for dischargingthe magnetic particles (iron powder) and oil that composes the MRFtogether with the magnetic particles is also discharged. It also causesa problem that the magnetic particles abrade the inner peripheralsurface of the rod guide 19.

In contrary to that, the outer peripheral surface of the piston rod 13shown in FIG. 6A is formed so that its surface roughness is small and sothe surface is highly smooth in forming the Ni plating film 24, so thatit is possible to prevent the magnetic particles from discharging to theatmosphere. Still more, because it is possible to suppress the magneticparticles from infiltrating between the inner peripheral surface of therod guide 19 and the outer peripheral surface of the piston rod 13, itis possible to suppress the inner peripheral surface of the rod guide 19from being abraded.

When the size of the magnetic particles contained in the MRF is around 2to 5 μm and precision (variation) of the thickness of the Ni platingfilm formed by means of the electroless Ni plating is ±10% for example,it is preferable to set the thickness of the Ni plating film 24 to beless than 20 μm in order to prevent the magnetic particles from beingdischarged to the atmosphere side (outside) through the sliding portionof the piston rod 13 and the rod guide 19.

It is noted that although no flowchart of a manufacturing method of thepiston rod 13 will be shown, the piston rod 13 is manufactured through astep of fabricating the rod base material 25, an electroless Ni platingstep and a heat treatment step in the same manner with the rod guide 19described later. Here, an electroless Ni plating solution utilizing thereducing effect of the hypophosphorus is suitably used in theelectroless Ni plating step. A method for forming the Ni plating film 24is substantially the same with that of the Ni plating film 22 explainedbefore. The heat treatment is carried out in the heat treatment step sothat the Vickers hardness of the Ni plating film 24 becomes 800 VHN ormore. It is possible to enhance productivity in manufacturing the pistonrod 13 here because no polishing process such as tape polishing isrequired to form the outer circumstance thereof to be true circle and toreduce the surface roughness to smooth the surface.

A non-magnetic material such as aluminum alloy is suitably used as therod base material 25. Preferably, the Ni plating film 24 has a thicknessof 10 μm or more and surface roughness of 0.1 to 1.5 in terms of a Rzvalue (0.01 to 0.15 in terms of a Ra value). Because the electroless Niplating step allows the Ni plating film 24 having the uniform thicknessto be formed, it is not necessary to carry out the rounding processafter the electroless Ni plating step in the manufacturing process ofthe piston rod 13. It is also possible to improve the abrasionresistance because the Ni plating film 24 may be hardened by the heattreatment. Still more, because the smooth surface having small surfaceroughness is formed by the electroless Ni plating step, it is notnecessary to carry out any polishing. It is also possible to suppressthe magnetic particles from infiltrating into the sliding surfacebetween the rod guide 19 and the piston rod 13 that is otherwise causedby irregularity of the surface. Accordingly, using such piston rod 13contributes a lot in the improvement of the durability of the rod guide19.

It is noted that the hardness of the Ni plating film 24 is preferable tobe 800 VHN or more in terms of the Vickers hardness. It allows very highabrasion resistance (durability) to be obtained. Still more, a chrome(Cr) plating film having the same nature and characteristics with the Niplating film 24 may be formed in stead of the Ni plating film 24 on thepiston rod 13. The Cr plating film may be formed by means ofelectrolytic plating. Because it is difficult to obtain the similaruniformity of thickness with that of the electroless Ni plating by theCr plating, polishing is normally carried out to obtain a desirablethickness and surface roughness.

Next, the rod guide of the variable damping-force damper of theembodiment will be explained with reference to FIGS. 6A and 6B.

FIG. 6B is a section view showing a structure of the rod guide of thevariable damping-force damper of the embodiment. Here, FIG. 6B is asection view taken along A-A in FIG. 6A that is a perpendicular partialsection view including the axial core of the piston rod 13 and shows thestructure of the piston rod 13 as well. The rod guide 19 shown in FIG.6C has the structure in which the electroless nickel plating film 32containing fluorine resin is formed on the inner peripheral surface(surface of a center hole) of a substantially cylindrical base materialportion 31. Note that the “nickel plating” will be described as “Niplating” and the electroless nickel plating film 32 containing fluorineresin” will be described as the “fluorine resin contained Ni platingfilm 32” hereinafter.

[Base Material Portion]

The variable damping-force damper 10 obtains the variable damping forceby changing the viscosity of the MRF within the communication hole 35 byapplying a magnetic field by the Ni plating film 22. Therefore, it isnecessary not to apply any magnetic field to the MRF at parts other thanthe communication hole 35. Therefore, the rod guide 19 must be made of anon-magnetic material (non-ferromagnetic material). The base materialportion 31 is also required to have mechanical characteristics, e.g.,Vickers hardness, yield point, tensile strength and others, desired as astructural part.

Therefore, various non-magnetic metallic materials such as aluminumalloy (aluminum and aluminum alloy) and stainless steel are suitably forthe base material portion 31. Preferably, aluminum alloy or the like isused in the present invention. The rod guide using the aluminum alloy orthe like will be described later as a modified example of the presentembodiment.

[Fluorine Resin Contained Ni Plating Film]

Tetrafluoride resin (PTFE) or the like is suitably used as the fluorineresin. The fluorine resin contained Ni plating film 32 has a coefficientof friction smaller than that of the electroless Ni plating filmcontaining no fluorine resin, so that friction between the piston rod 13and the rod guide 19 becomes small and sliding resistance may bereduced. Therefore, even if a side force acts on the piston rod 13 andthe piston rod 13 is pressed against the rod guide 19, it becomespossible to suppress an increase of the sliding resistance between thepiston rod 13 and the rod guide 19 and to reduce a drop of theoperational accuracy with respect to an input signal.

The hardness of the fluorine resin contained Ni plating film 32 ispreferable to be 360 VHN or more in terms of the Vickers hardness sothat the fluorine resin contained Ni plating film 32 can have effectiveabrasion resistance against the magnetic particles when the magneticparticles infiltrate into the gap between the piston rod 13 and the rodguide 19. A thickness of the fluorine resin contained Ni plating film 32is preferable to be 10 μm or more even though it depends on a size andan usable life of the variable damping-force damper 10.

[Manufacturing Method and Manufacturing Conditions of Rod Guide]

FIG. 7 is a flowchart showing a manufacturing method of the rod guide.The rod guide 19 is manufactured through a process including a basematerial portion fabricating step S1 of forming the base materialportion 31, an electroless Ni plating step S2 of forming the fluorineresin contained Ni plating film 32 (before heat treatment) on thesurface of the base material portion 31 formed in the base materialportion fabricating step S1 that slides relative to the piston rod 13and a heat treatment step S3 of implementing the heat treatment on thefluorine resin contained Ni plating film 32 (before the heat treatment)formed by the electroless Ni plating step S2.

The base material portion fabricating step S1 is carried out bymechanically machining a predetermined metal ingot (or a columnar steelbar) into a desirable cylindrical shape. The next s2 is carried out byusing a plating solution prepared by adding and mixing an adequateamount of fluorine resin into an electroless Ni plating solutionutilizing the reducing effect of the hypophosphorus and phosphorus andthe fluorine resin are codeposited in precipitating Ni. It is possibleto form the fluorine resin contained Ni plating film 32 (before the heattreatment) having an uniform thickness of a target thickness ±3 μm or soby the electroless Ni plating process as described above. Therefore, itis not necessary to adjust the thickness of the fluorine resin containedNi plating film 32 by polishing such as honing after the electroless Niplating step S2. Still more, it is not necessary to polish the surfacebecause the electroless Ni plating process allows the surface roughnessof the fluorine resin contained Ni plating film 32 to be kept low. Thus,it is possible to keep the production cost low.

The fluorine resin contained Ni plating film 32 (before the heattreatment) formed by the electroless Ni plating step S2 isnoncrystalline and its hardness is not so high. Then, the fluorine resincontained Ni plating film 32 (before the heat treatment) is hardened bychanging its nature from noncrystalline to crystalline by the heattreatment step S3. “Changing to crystalline” here means to “form acrystalline phase of Ni₃P.” The heat treatment conditions are set withinsuch a range that the crystalline phase of Ni₃P is formed and nofluorine resin melts, liquating out of the fluorine resin contained Niplating film 32 and aggregating together. Thus, the fluorine resincontained Ni plating film 32 (after the heat treatment) having amicrostructure in which the crystal phase of Ni₃P and the fluorine resinare homogeneously dispersed and the enhanced hardness is obtained. It isnoted that the base material portion 31 is also heated at the same timein the heat treatment step S3, so that heat treatment temperature in theheat treatment step S3 must be decided by considering thermalcharacteristics such as fusion point, changes of mechanicalcharacteristics caused by heating and others of the material used forthe base material portion 31. The heat treatment step S3 is carried outat about 300° C. for about one hour for example.

<Packing>

The rod guide 19 is provided with a packing 26 on the side of the piston16 as a sealing member for preventing the MRF from leaking out of thecylinder tube 12. The packing 26 is made of a rubber-base polymermaterial and is provided preferably on the side closer to the piston 16than the fluorine resin contained Ni plating film 32. Thus, the packing26 can prevent the magnetic particles from infiltrating into the slidingsurface between the rod guide 19 and the piston rod 13 and to keep theabrasion of the both sliding surfaces of the rod guide 19 and the pistonrod 13.

Next, a first modified example of the rod guide of the embodiment willbe explained.

FIGS. 8A and 8B are section views showing a structure of the rod guideof the first modified example of the present embodiment. FIGS. 8A and 8Bare drawn similarly to FIGS. 6A and 6B. FIG. 8A is a perpendicularsection view including the axial core of the piston rod and FIG. 8B is asection view taken along A-A in FIG. 8A. A rod guide 19A shown in FIGS.8A and 8B has a structure in which an Ni plating film 32A is formed bymeans of the electroless Ni plating on the inner peripheral surface(surface of the center hole) of the substantially cylindrical basematerial portion 31.

[Base Material Portion]

Al alloy or the like, i.e., pure Al or Al alloy, is used for the basematerial portion 31 of the rod guide 19A. The pure Al containsinevitable impurities. The aluminum alloy contains Al as its maincomponent, alloy ingredients such as copper (Cu), manganese (Mn),silicon (Si), magnesium (Mg), zinc (Zn), nickel (Ni) or the like andinevitable impurities. One of merits of using the aluminum alloy or thelike for the base material portion 31 is that because the aluminum alloyor the like is inexpensive and is readily workable, it enables onehaving a desirable shape to be readily manufactured and hence excels inproductivity. The aluminum alloy having well-balanced desired mechanicalcharacteristics is suitably used for the base material portion 31 andspecifically, JIS A6061 T6 series aluminum alloy may be used forexample.

Another merit of using the aluminum alloy or the like for the basematerial portion 31 is that when the variable damping-force damper 10 isbiased by the side force or the like, deformation (elastic deformation)of the aluminum alloy or the like can suppress an increase of thesurface pressure at the surface where the rod guide 19A contacts withthe piston rod 13 and can suppress an increase of the slidingresistance.

This effect will be explained with reference to FIGS. 9A and 9B. Notethat a reference is made also to FIGS. 6A and 6B appropriately. FIGS. 9Aand 9B diagrammatically show states in which the piston rod is pressedby the side force locally against the rod guide in a radial direction.FIG. 9A shows a case when Young's modulus of the base material portionis large (comparative example not belonging to the present invention andFIG. 9B shows a case when the Young's modulus of the base material ofthe rod guide is small (embodiment belonging to the invention). In orderto signify only an effect brought about by the base material of the rodguide, FIGS. 9A and 9B show the rod guide composed of only the basematerial portion and the piston rod whose structure is simplified. Notethat no reference numeral is denoted each component in FIGS. 9A and 9B.

The axial core of the piston rod coincides substantially with the axialcore of the center hole of the rod guide in the state in which no sideforce F acts on the piston rod (see FIGS. 6A and 6B) and the surfacepressure of the sliding surface of the piston rod and the rod guide inthis state (referred to as a “surface pressure in the normal state”hereinafter) may be considered to vary less.

However, if the side force F acts on the piston rod as shown in FIGS. 9Aand 9B, the piston rod moves in the radial direction. If the rod guideis immovable here, a part of the outer peripheral surface of the pistonrod [a belt-like area whose longitudinal direction is in parallel withthe axial core direction of the piston rod (referred to as a “pressedarea” hereinafter)] is pressed against a part of the inner peripheralsurface of the rod guide.

When the base material portion of the rod guide is made of a material,e.g., stainless steel or the like whose Young's modulus is large andthat is hardly deformed by an external force, at this time, the rodguide receives a pressing force F from the piston rod by the narrowpressed area because the base material portion does not deform as shownin FIG. 9A. Then, a surface pressure F1 of the pressed area becomes verylarge as compared to the surface pressure in the normal time, increasingthe sliding resistance between the piston rod and the rod guide.Operational accuracy of the variable damping-force damper with respectto an input signal (input of electric current to the magnetic coil 52)drops due to the increased sliding resistance in such a state. Theabrasion in the pressed area also tends to advance in the both sides ofthe piston rod and rod guide.

When the base material portion is made of a material such as thealuminum alloy that is classified as having smaller Young's modulusamong metal materials in contrary, a part of the rod guide pressed bythe piston rod deforms elastically (dents) as shown in FIG. 9B. Thereby,a surface pressure F2 of a pressed area becomes smaller than the surfacepressure F1 in the non-deformed state because the pressing force isrelaxed and the pressed area is enlarged. Therefore, it is possible tosuppress the increase of the sliding resistance between the piston rodand the rod guide and to keep the drop of the operational accuracy lowwith respect to the input signal in the case of the structure shown inFIG. 9B as compared to the case of the structure shown in FIG. 9A. Stillmore, it is possible to suppress the advance of the abrasion of thepressed area on the both sides of the piston rod and rod guide. It isnoted that because the deformation of the base material portion shown inFIG. 9B is elastic deformation, the base material portion returns to theoriginal state when the side force F (the pressing force from the pistonrod) is released.

The Young's modulus of Al increases by alloying in general and theeffect of suppressing the increase of the surface pressure of thepressed area by the elastic deformation described above decreases if theYoung's modulus increases. Meanwhile, the rod guide 19A is required tohave mechanical characteristics for supporting the piston rod 13.Therefore, it is preferable to consider their balance in selecting thealuminum alloy or the like to be used for the base material portion 31.

[Ni Plating Film]

When the base material portion 31 of the rod guide 19A is made of thealuminum alloy or the like and when the base material portion 31 slidesdirectly relative to the piston rod 13, the aluminum alloy or the liketends to abrade because its hardness is small and there is a possibilitythat the abrasion of the sliding surface of the rod guide 19A isaccelerated when the magnetic particles (iron powder) contained in theMRF enter the gap between the piston rod 13 and the rod guide 19A inparticular. If the abrasion of the sliding surface of the rod guide 19Aadvances, there is a possibility that the magnetic particles are apt toenter between the gap between the piston rod 13 and the rod guide 19A,accelerating the abrasion of the sliding surface of the rod guide 19Aeven more.

Still more, if the abrasion of the sliding surface of the rod guide 19Aadvances and the clearance between the piston rod 13 and the rod guide19A is widened, there is a possibility that the piston rod 13 becomesbumpy (wobbles) when the variable damping-force damper 10 is driven. Thewobble of the piston rod 13 destabilizes the contact of the slidingsurface of the piston rod 13 with the rod guide 19A and may cause biasedabrasion. Still more, because the piston 16 tends to slide relative tothe cylinder tube 12 in a state in which the axial core direction of thepiston 16 crosses over the axial core direction of the cylinder tube 12with a certain angle, the piston 16 and the cylinder tube 12 tend tocause biased abrasion, causing drops of the accuracy in controlling thedamping force and of the durability.

Then, the Ni plating film 32A is provided on the inner peripheralsurface (the inner surface of the center hole) of the rod guide 19A inorder to enhance the abrasion resistance of the sliding surface of therod guide 19A without hampering the effect of suppressing the increaseof the surface pressure of the sliding surface between the piston rod 13and the rod guide 19A by using the aluminum alloy or the like for thebase material portion 31. Preferably, the hardness of the Ni platingfilm 32A is 360 VHN or more in terms of Vickers hardness from an aspectof effectively assuring the abrasion resistance against the magneticparticles.

While the Ni plating film 32A may be formed through the electroless Niplating step S2 and the heat treatment step S3 (see FIG. 7appropriately), it is arbitrary whether or not to add fluorine resin tothe electroless Ni plating solution utilizing the reducing effect of thehypophosphorus in the electroless Ni plating step S2 here. That is,although the Ni plating film 32A must contain phosphorus, it may or maynot contain fluorine resin. The heat treatment condition of the heattreatment step S3 may be changed depending on whether or not fluorineresin is contained in the Ni plating film 32A.

It is preferable to use the fluorine resin contained Ni plating film asthe Ni plating film 32A because the sliding surface of the piston rod 13and the rod guide 19A may be kept even low due to the small coefficientof friction of the fluorine resin contained Ni plating film.

A thickness of the Ni plating film 32A is set so that the function ofsuppressing the increase of the surface pressure of the sliding surfaceof the piston rod 13 and the rod guide 19A is fully exhibited and sothat the Ni plating film 32A does not peel off or crack even when therod guide 19A receives the pressing force from the piston rod 13 and thebase material portion 31 elastically deforms. Still more, while the Niplating film 32A is not what does not abrade at all and its thickness isthinned by temporal abrasion, the initial thickness of the Ni platingfilm 32A is set so that the Ni plating film 32A remains even when apredetermined usable life (usable years) elapses. Although the thicknessof the Ni plating film 32A varies depending on the size, usable life andothers of the variable damping-force damper 10, it is preferable to be10 μm or more.

[Influence on Heat Treatment of Ni Plating Film on Al Alloy or the LikeUsed for Base Material]

The aluminum alloy or the like composing the base material portion 31 isheated by the heat treatment temperature during the heat treatment stepS3 of the Ni plating film 32A. Because the aluminum alloy or the like isa material whose fusion point low among metallic materials, it isnecessary to set the heat treatment conditions (temperature, time andothers) so that the aluminum alloy or the like does not melt, soften ordeforms.

The precipitation state of the alloy ingredients largely influences onthe mechanical characteristics of the aluminum alloy.

Therefore, it is preferable to use one in which the alloy ingredientsprecipitate homogeneously in using the aluminum alloy for the basematerial portion 31. If the alloy ingredients precipitate homogeneously,it is possible to suppress moves caused by dislocation of various metalatoms composing the aluminum alloy and to keep the mechanicalcharacteristics. To that end, it is preferable to balance thecomposition and others of the aluminum alloy and the heat treatmentconditions of the Ni plating film 32A so that the precipitation state ofthe alloy ingredients in the aluminum alloy does not change as much aspossible by the heat treatment step S3 of the Ni plating film 32A. Notethat it is also possible to arrange so that the alloy ingredientsprecipitate homogeneously in the aluminum alloy during the heattreatment process of the Ni plating film 32A.

A solid solution state of the alloy ingredients also largely influenceson the mechanical characteristic of the aluminum alloy. Therefore, it ispreferable to arrange so that the alloy ingredients precipitated in thealuminum alloy do not become the solid solution state in Al (a state inwhich the alloy ingredients melt homogeneously into Al) by the heattreatment step S3 of the Ni plating film 32A in using the aluminum alloyin which the alloy ingredients precipitate. It allows the mechanicalcharacteristics of the aluminum alloy to be kept.

Note that it is also preferable to suppress segregation of the alloyingredients and to prevent the mechanical characteristics from droppingin the heat treatment step S3 of the Ni plating film 32A in using thealuminum alloy for the base material portion 31. The segregation refersto inhomogeneous precipitation of the alloy ingredients and includescases when the alloy ingredients that have been homogeneouslyprecipitated beforehand by an ageing treatment in the manufacturingprocess of the aluminum alloy distribute inhomogeneously due toaggregation or the like and when the alloy ingredients that have beenhomogeneously solid-soluble in Al are precipitated inhomogeneously. Notethat the segregation of the alloy ingredients may be readily verifiedfrom a result or the like of plane analysis performed on a distributionof elements by means of a SEM-EDX or the like for example. Whether ornot the segregation of the alloy ingredients is occurring by the heattreatment step S3 may be determined from changes of texture of thealuminum alloy before and after the heat treatment step S3 and from thedrop of the mechanical characteristics.

Still more, because the aluminum alloy or the like is a heat-treatedmaterial (whose mechanical characteristics are adjusted) by the heattreatment, it is necessary to avoid a case of dropping the mechanicalcharacteristics or losing the desirable mechanical characteristics ofthe aluminum alloy or the like composing the base material portion 31 bythe heat treatment step S3. When the aluminum alloy is used for the basematerial portion 31 and the Ni plating film 32A is heat-treated attemperature around an annealing condition of the aluminum alloy forexample, there is a possibility that the aluminum alloy is softened andloses the mechanical characteristics required as the rod guide 19A.Therefore, it is preferable to set the heat treatment temperature of theheat treatment step S3 at temperature by which the aluminum alloy or thelike composing the base material portion 31 is not annealed. In otherwords, it is preferable to select the aluminum alloy or the like, usedas the base material portion 31, having the composition that causessubstantially no change in the mechanical characteristics under the heattreatment condition by which the Ni plating film 32A would have thedesirable hardness.

Next, a second exemplary modification of the rod guide of the presentembodiment will be explained.

FIGS. 10A and 10B are both section views showing a structure of the rodguide. FIG. 10A is a perpendicular section view including the axial coreof the piston rod and FIG. 10B is a section view taken along A-A in FIG.10A. A rod guide 19B shown in FIGS. 10A and 10B has a structure in whichan Ni plating film 33A having a two-layered structure (referred to as a“two-layered Ni plating film 33” hereinafter) is formed on the innerperipheral surface (surface of the center hole) of the substantiallycylindrical base material portion 31. The two-layered plating film 33 iscomposed of a first electroless Ni plating layer 36 (described as the“first plating layer 36” hereinafter) formed on the surface of the basematerial portion 31 and containing phosphorus (P) and a secondelectroless Ni plating layer 37 (described as the “second plating layer37” hereinafter) formed on the surface of the first plating layer 36 andcontaining fluorine resin.

The first plating layer 36 contains no fluorine resin, so that it may beformed by using the electroless Ni plating solution utilizing thereducing effect of the hypophosphorus. Meanwhile, the second platinglayer 37 is the same with the fluorine resin contained Ni plating film32 provided on the rod guide 19 and a forming method thereof conforms tothe method for forming the fluorine resin contained Ni plating film 32as described above. It is noted that a manufacturing method of thetwo-layered plating film 33 may be carried out in order from anelectroless Ni plating process of the first plating layer 36, a heattreatment, an electroless Ni plating process and a heat treatment or inorder from the electroless Ni plating process of the first plating layer36, the electroless Ni plating process and the heat treatment.

When the two-layered plating film 33 is used, its manufacturingcondition is adjusted so that Vickers hardness of the first platinglayer 36 becomes larger than that of the second plating layer 37. Itallows the drop of the operational accuracy with respect to the inputsignal to be kept low because the sliding resistance is kept low sincethe second plating layer 37 containing the fluorine resin slidesrelative to the piston rod 13 in an initial stage of usage andsuppresses an increase of the sliding resistance between the piston rod13 and the rod guide 19B even in a state when a side force or the likeacts on the piston rod 13.

Then, the second plating layer 37 abrades with time and the firstplating layer 36 comes to slide relative to the piston rod 13, aclearance between the piston rod 13 and the rod guide 19B is widened.Although there is a possibility that the magnetic particles tend toenter the widened clearance, advancing the abrasion of the slidingsurface, the first plating layer 36 that is superior than the secondplating layer 37 in terms of abrasion resistance is exposed at thistime, so that the first plating layer 36 suppresses the advance ofabrasion of the rod guide 19B and maintains the durability. It is alsopossible to suppress an increase of a surface pressure (increase of thesliding resistance) of a contact surface between the piston rod 13 andthe rod guide 19B by utilizing elastic deformation of the aluminum alloyor the like even when the side force or the like acts by using the basematerial portion 31 composed of the aluminum alloy or the like is usedfor the rod guide 19B.

[Embodiments]

Firstly, a result of a durability test of the Ni plating film formed onthe inner peripheral surface of the cylinder tube will be explained byassuming a case when the variable damping-force damper is used as asuspension of a vehicle.

<Fabrication of Variable-damping Force Damper>

[Variable-damping Force Damper of First Embodiment]

A Ni plating film was formed on an inner peripheral surface of acylinder base material made of a steel pipe for general structure sothat its average thickness becomes 40 μm with forming speed of 0.1μm/min. by using an electroless Ni plating solution and was thenheat-treated in 315 to 327° C. for two hours. Thus, the cylinder tubehaving the Ni plating film (Ni—P plating film) whose Vickers hardness is900 VHN and whose surface roughness (average height of+point Rz) is 1.3μm and inner diameter of about φ 46 mm (tolerance: less than 5 μm) wasobtained. It is noted that the Vickers hardness and the surfaceroughness of the Ni plating film were measured respectively by means ofa commercially available Vickers hardness tester and a surface roughnessmeter by using a Ni plating film formed by simultaneously treating aflat plate made of a steel plate for general structure, i.e. the samematerial with the cylinder base material. It was confirmed that thecrystal phase of Ni₃P had been formed when a phase of the Ni platingfilm formed on the flat plate was identified by XRD.

A piston was prepared so that it has the structure shown in FIG. 2, itsdiameter is about 45.9 mm so that the clearance between the slidingportion of the piston and the inner peripheral surface of the cylindertube becomes about 50 μm, its length of the piston sliding portion L is13 mm and it has a piston ring made of S25C (carbon steel for mechanicalstructure). A free piston made of aluminum and is provided with anO-ring that slides relative to the Ni plating film was prepared. Thevariable damping-force damper (see FIG. 1 appropriately) of the firstembodiment was fabricated by using these parts. Then, the first andsecond oil chambers of the variable damping-force damper were filledwith the MRF.

[Variable-damping Force Damper of Second Embodiment]

A Ni plating film was formed on an inner peripheral surface of acylinder base material that is the same one with the cylinder basematerial used in the fabrication of the variable damping-force damper ofthe first embodiment so that its average thickness becomes 40 μm withforming speed of 0.1 μm/min. by using a plating solution in which BNpowder is dispersed in the electroless Ni plating solution used in thefirst embodiment and was then heat-treated in 315 to 327° C. for twohours. Thus, the cylinder tube having the Ni plating film (Ni—P+BNplating film) whose Vickers hardness is 900 VHN and whose surfaceroughness (average height of+point Rz) is 1.3 μm and an inner diameterof about φ46 mm (tolerance: less than 5 μm) was obtained. The variabledamping-force damper of the second embodiment was fabricated by usingthe cylinder tube and a piston equivalent to the piston used in thefabrication of the variable damping-force damper of the firstembodiment.

[Variable-damping Force Damper of Comparative Example]

A Cr plating film was formed on an inner peripheral surface of acylinder base material that is the same one with the cylinder basematerial used in the fabrication of the variable damping-force damper ofthe first embodiment so that its average thickness becomes 100 μm bymeans of electroplating. The Cr plating film thus formed had 900 to 1000VHN of Vickers hardness. Successively, homing was implemented on the Crplating film so that an inner diameter of the cylinder tube becomesabout φ 46 mm (tolerance: less than 5 μm). The variable damping-forcedamper of the comparative example was fabricated by using this cylindertube and the piston equivalent to the piston used in the fabrication ofthe variable damping-force damper of the first embodiment.

<Condition of Durability Test>

Conforming to the method of the durability test previously explainedwith reference to FIG. 3, a test was carried out on the variabledamping-force damper of the first embodiment by reciprocating thesliding portion of the piston within the piston sliding range of 70 mmin terms of the length of the cylinder tube in the axial core directionin the state when the certain side force F (load) is applied in oneradial direction (perpendicularly downward direction) of the piston.After ending the test, the inner diameters of the cylinder tube 12 inthe F-direction, in the 45° direction, in the 90° direction and in thepiston rod 135° direction (see FIG. 4 appropriately) were measured atpredetermined intervals in the axial core direction of the cylinder tubeby using the commercially available cylinder gauge and the like(measured minimum unit: 1 μm, measuring error; 1 μm.

<Result of Durability Test>

[Relationship Between Thickness of Ni Plating Film and Length of PistonSliding Portion L]

FIG. 11 is a graph showing changes of each inner diameter of thecylinder tube in the axial core direction after the durability test ofthe variable damping-force damper of the first embodiment. In FIG. 11,variation of each inner diameter after the durability test representsthe abrasion Δt of the Ni plating film. It can be seen that a maximumvalue in the changes of the inner diameter of the cylinder tube withinthe piston sliding range is about 32 μm in the inner diameter in theF-direction. There is no big change before and after the durability testin the inner diameter in the 90° direction. It happens possibly becausethe sliding surface of the piston does not contact substantially withthe inner peripheral surface of the cylinder tube in the direction ofthe inner diameter of 90°. It also suggests that substantially noabrasion of the Ni plating film otherwise caused by the magneticparticles contained in the MRF has occurred.

As shown in FIG. 11, there appears a tendency that the changes of theinner diameters of a center area of the piston sliding range are largerthan the changes of the inner diameters in edge areas (the abrasion ofthe Ni plating film 22 increases). It happens possibly because slidingspeed at the center area is high and because the whole length of thepiston passes there.

When a plurality of variable damping-force dampers of the firstembodiment was fabricated and the durability test described above wascarried out on each variable damping-force damper, an average value ofthe abrasion Δt of the Ni plating film was 30 μm. Then, based on theabrasion Δt of the Ni plating film=30 μm, a relationship between thelength of the piston sliding portion L and a least minimum thicknessrequired for the Ni plating film was found.

The surface pressure PMEAN between the piston ring and the Ni platingfilm in this durability test was calculated as 6.73 Mpa from Equations 1through 6 described above. Note that values of the piston ring and theNi plating film were set as follows: the Young's modulus of theelectroless Ni plating step S25C used for the piston ring E1=210 Gpa andits Poisson's ratio v1=0.29, the Young's modulus of the Ni plating filmE2=219 Gpa and its Poisson's ratio v2=9.31 and the radius of the pistonring R1=22.95 mm and the radius of the Ni plating film R2=23 mm.

FIG. 12 is a graph showing the relationship between the abrasion Δt ofthe Ni plating film and the surface pressure PMEAN. Assuming that theabrasion Δt of the Ni plating film is a linear function of the surfacepressure PMEAN from Eq. 7 described above, its coefficient K was foundto be “K=4.46 [μum/Mpa].” The relationship between the abrasion Δt ofthe Ni plating film and the length of the piston sliding portion L wasfound from Eq. 8 described above by using this coefficient K. FIG. 13 isa graph showing the result thereof. From the result shown in FIG. 13, itwas found that when the length of the piston sliding portion L is 50 mm,the abrasion Δt of the Ni plating film is 15 μm, i.e., that the requiredleast minimum thickness of the Ni plating film must be 15 μm.

It is noted that the result shown in FIG. 12 is that of the case whenthe Ni—P plating film was used as the Ni plating film. The requiredleast minimum thickness of the Ni plating film may be found even whenthe Ni plating film containing various additional ingredients describedabove is used by carrying out the similar test.

[Aggression of Ni Plating Film Against Piston Ring]

When the abrasion of the sliding surface in the piston ring was measuredafter the durability test of the variable damping-force damper of thefirst embodiment, the maximum abrasion was about 56 μm in average.Accordingly, a total abrasion of the Ni plating film is about 86 μm.FIG. 14 shows a graph representing the total abrasion.

A durability test was carried out on the variable damping-force damperof the comparative example under the same conditions with the durabilitytest on the variable damping-force damper 10 of the first embodiment. Asa result, the abrasion of the Cr plating film was about 8 μm in theinner diameter of the F-direction. Meanwhile, the abrasion of thesliding surface of the piston ring was about 126 μm. FIG. 14 also showsa graph showing this total abrasion.

As it is apparent from FIG. 14, although the abrasion of the Cr platingfilm itself is very small, the abrasion of the piston ring that is asliding mating member is large in the variable damping-force damper ofthe comparative example. In contrary to that, because the abrasion ofthe piston ring is kept small in the variable damping-force damper ofthe first embodiment, it was confirmed that the aggression of the Niplating film against the piston ring became low. That the total abrasionof the piston ring and the Ni plating film is low indicates that changeof the clearance between the inner peripheral surface of the cylindertube and the outer peripheral surface of the piston is small. Therefore,it was confirmed that the variable damping-force damper of the firstembodiment can suppress the MRF from freely moving between the first andsecond oil chambers through the clearance part for a long period of timeand can maintain the damping force for a long period of time.

[Reduction of Friction of Ni Plating Film by Adding Ingredients]

Magnitude of the friction between the cylinder tube and the piston whenthe magnitude of the side force F is respectively set at 0 kgf, 10 kgf(=98.07 N) and 20 kgf (=196.13 N) was measured on the variabledamping-force dampers of the first and second embodiments. FIG. 15 is agraph showing its result. As shown in FIG. 15, the friction is small inthe variable damping-force damper of the second embodiment as comparedto that of the variable damping-force damper of the first embodiment andthe effect of reducing the coefficient of friction by BN was confirmed.It becomes possible to reduce not only the abrasion of the Ni platingfilm but also the abrasion of the piston ring by thus reducing thefriction between the cylinder tube and the piston.

Next, a result of evaluation of the friction between the piston rod andthe rod guide when the side force is added to the variable damping-forcedamper of the embodiment will be explained.

FIG. 16 is a section view of the variable damping-force damperschematically showing a testing method for measuring the frictionbetween the piston rod and the rod guide. This friction evaluation testwas carried out by preparing a rod guide in which fluorine resin hardalumite (comparative example) , a fluorine resin contained Ni platingfilm (third embodiment) and a fluorine resin bearing (comparativeexample) are provided respectively on the sliding surface of the rodguide relative to the piston rod and by measuring the friction when thepiston rod is slid relative to the rod guide in a state in which nopiston is attached as shown in FIG. 16, no MRF is filled within thecylinder tube and the side force of a predetermined magnitude is actedon a part of the cylinder tube where the rod guide is positioned.

It is noted that the aluminum alloy was used for the base materialportion composing the rod guide to evaluate the effect of suppressing anincrease of the friction of various films provided on the slidingsurface of the rod guide against the side force. The piston rod made oflow-carbon steel, e.g., S45C, and having a hard Cr plating film providedon the surface thereof was used. This test was carried out under thefollowing conditions:

Lubricant environment (sliding surface of the piston rod and the rodguide): dry (no lubricant is used)

Magnitude of side force: three magnitudes of 0, 10 and 15 kgf (=0, 98and 196 N)

Sliding speed: 0.005 m/sec.

Sliding displacement: ±5 mm

FIG. 17 is a graph showing measured test results. As it is apparent fromFIG. 17, the fluorine resin contained Ni plating film significantlysuppresses an increase of the friction as compared to the fluorine resinhard alumite. It was confirmed from the fact that the variabledamping-force damper whose operation accuracy with respect to an inputsignal drops less even when the sliding surface acts may be realized. Itis noted that although an absolute value of the friction is least whenthe fluorine resin bearing is used, the fluorine resin bearing is notsuitable for the variable damping-force damper using the MRF because itis soft and is inferior in terms of the abrasion resistance as explainedin the background art of the invention.

It is apparent from an aspect of material property that the fluorineresin contained Ni plating film is superior in terms of the abrasionresistance than the fluorine resin hard alumite because the Vickershardness of the fluorine resin contained Ni plating film is about 1.5times of that of the fluorine resin hard alumite. Accordingly, it may bedetermined to be preferable to use the fluorine resin contained Niplating film rather than the fluorine resin hard alumite.

It is noted that although the mode described above is the best mode forcarrying out the invention, it is not intended to limit the invention tosuch mode. Accordingly, the mode for carrying out the invention may bevariously modified within a scope in which the subject matter of theinvention is not changed.

1. A variable damping-force damper, comprising: a cylinder tube filledwith a working fluid that is a magnetic fluid or a magneto-rheologicalfluid containing magnetic particles; and a piston that partitions aninside of the cylinder tube into first and second chambers, has acommunication hole for circulating the work fluid between the first andsecond chambers and has an electromagnetic coil that applies a magneticfield to the work fluid within the communication hole; the variabledamping-force damper controlling a damping force by changing viscosityof the work fluid within the communication hole by feeding power to theelectromagnetic coil; wherein the variable damping-force damper has asliding surface that slides under an influence of the magnetic particlesduring contraction; the sliding surface has a heat-treated nickelplating film on the surface thereof; wherein the sliding surface is anouter peripheral surface of the piston or an inner peripheral surface ofthe cylinder tube and the Ni plating film is provided at least on theinner peripheral surface of the cylinder tube; wherein a thickness ofthe Ni plating film is 15 μm or more when a length of sliding portion ofthe piston in an axial core direction is 50 mm; and when the length ofthe sliding portion is shorter than 50 mm, the thickness of the Niplating film is set such that the shorter the length of the slidingportion, the thicker the thickness of the Ni plating film becomes beyond15 μm.
 2. The variable damping-force damper according to claim 1,wherein the Ni plating film contains phosphorus and one or a pluralityof elements or chemical compounds selected from boron, tungsten, boronnitride and silicon carbide.
 3. The variable damping-force damperaccording to claim 1, wherein the Vickers hardness of the Ni platingfilm formed on the inner peripheral surface of the cylinder tube is 800VHN or more.
 4. The variable damping-force damper according to claim 1,further comprising a piston rod whose one end is attached to the pistonand whose other end extends to an outside of the cylinder tube; whereinthe cylinder tube has a cylindrical rod guide section disposed at oneend of the tube so that the piston rod is inserted through the rodguide; and the Ni plating film is also provided on the outer peripheralsurface of the piston rod.
 5. The variable damping-force damperaccording to claim 1, further comprising a piston rod whose one end isattached to the piston and whose other end extends to an outside of thecylinder tube; wherein the cylinder tube has a cylindrical rod guidesection disposed at one end of the tube so that the piston rod isinserted through the rod guide; and at least the rod guide has aheat-treated electroless Ni plating film that contains phosphorus andfluorite resin on an interfacial sliding surface of an outer peripheralsurface thereof.
 6. The variable damping-force damper according to claim5, wherein the rod guide has a base material portion made of aluminum oraluminum alloy; the electroless Ni plating film containing phosphorusand fluorite resin is formed on a predetermined surface of the basematerial portion; and the piston rod slides relative to the electrolessNi plating film containing phosphorus and fluorite resin.
 7. Thevariable damping-force damper according to claim 6, wherein the basematerial portion is made of the aluminum alloy and precipitates alloyingredients other than aluminum.
 8. The variable damping-force damperaccording to claim 6, wherein the electroless Ni plating film containingphosphorus and fluorite resin has: a first electroless Ni plating layerformed on the surface of the base material portion and containingphosphorus; and a second electroless Ni plating layer formed on thesurface of the first electroless Ni plating layer and containingphosphorus and fluorine resin; wherein the first electroless Ni platinglayer has a hardness that is harder than that of the second electrolessNi plating layer.
 9. The variable damping-force damper according toclaim 5, further comprising a sealing member provided in the rod guideto prevent the working fluid from leaking out of the cylinder tube;wherein the sealing member is provided on the side closer to the pistonrather than the electroless Ni plating film containing phosphorus andfluorite resin in the axial core direction of the cylinder tube.
 10. Thevariable damping-force damper according to claim 5, wherein the pistonrod is provided with an electroless Ni plating film or a Cr plating filmon the sliding surface that slides relative to the rod guide; whereinthe electroless Ni plating film or the Cr plating film on the piston rodhas a thickness of 10 μm or more and a surface roughness of 0.1 to 1.5in terms of a Rz value.
 11. The variable damping-force damper accordingto claim 1, wherein when the length of the piston sliding portion isshorter than 50 mm, the thickness of the Ni plating film is set so thatthe shorter the length of the piston sliding portion L becomes below 50mm, the thickness of the Ni plating film is increased so that it becomesinversely proportional to a square root of the length of the pistonsliding portion.